Veeco Dimension 3100, Manual

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TappingMode AFM
Advanced TappingMode AFM Operation
Rev. D Dimension 3100 Manual 147
The Setpoint parameter defines the desired voltage for the feedback loop. The Setpoint voltage is
constantly compared to the present RMS amplitude voltage to calculate the desired change in the
piezo position. When the gain values are high, as they should be when the Data type is set to
Height , the Z piezo position changes to keep the amplitude voltage close to the Setpoint ; therefore,
the oscillation amplitude remains nearly constant.
As discussed above, changing the Setpoint alters the response of the cantilever oscillation and
changes the amount of force applied to the sample surface.
The force applied by the tip on the sample increases as the setpoint is decreased. To minimize the
force on the sample:
• Increase the setpoint using the right arrow key on the computer keyboard until the
tip is no longer interacting with the surface. This can be observed by monitoring the
trace and retrace lines in
View > Scope Mode , and by monitoring the Z Center
Position display in the Image Monitor. When the tip is no longer interacting with
the surface, the Z Center Position will move to the retracted limit.
• Slowly decrease the setpoint using the left arrow key on the keyboard until features
appear in the trace and retrace lines in
View > Scope Mode .
The setpoint can also be adjusted by using View > Force Mode > Calibrate . Force calibration
mode is used to compare the amplitude of the cantilever oscillation on the surface to the free-air
amplitude. Other uses of Force Calibration include characterizing the forces on the cantilever tip,
diagnosing the performance, and calibrating the RMS amplitude voltage as a function of the
oscillation amplitude of the cantilever. The use of Force Calibration is described in
Chapter 13 .
9.4.8 Surface Tune
It is often desirable to check the cantilever tune after engaging. You will notice that the Cantilever
Tune icon is grayed out when the tip is engaged. However, you can still access the
Cantilever Tune panel by selecting View > Sweep > Cantilever Tune .
A warning appears indicating that cantilever tuning while engaged can result in damage to the tip or
sample. This warning prompts you to enter a
Tip offset . This is the distance above the sample that
the tip will be raised before displaying the tuning spectrum. Use a minimum Tip offset of 100 -
200nm .
Observe the plot of amplitude vs. driving frequency to see if the cantilever tuning curve shifted
during the engagement. The drive frequency can be adjusted to compensate for such a shift by using
the
Offset command located above the Cantilever Tune plot. The phrase response curve can be
adjusted as well.